Our broad goals are to understand the regulation, lineage determination, and plasticity that the hemopoietic microenvironment imposes on the development of B-lineage lymphocytes. the normal production of newly formed B-cells seems highly controlled during ontogeny and is particularly vulnerable to dysregulation by certain diseases and treatments. several convincing studies, primarily using cell clones or lines in vitro, imply that a type of mesenchymal accessory cell, referred to as a "stromal cell", provides growth and differentiation stimuli as well as a structural framework for early B-cells and other blood cell lineages. However, work to date with cultured stromal cells is insufficient to conclude (a) what the "stromal cell" equivalent is in vivo, (b) what cytokines are produced in different physiological circumstances, and (c) the extent of heterogeneity that exists among normal stromal cells. Our focus is committed to understanding the biology of primary stromal cells, either freshly isolated or with minimal culture passage, and to relating our data to the many lines of stromal cells established in different laboratories. Three cytokines which may be pivotal to B-cell formation, interleukin-7 (IL-7), stem cell factor (SCF), and insulin-like growth factor-1 (IGF-1) are expressed by primary cultured stromal cells during lymphopoiesis. The proposed studies test the hypothesis that differential cytokine production by marrow stromal cells regulates B-lymphopoiesis, especially during marrow regeneration. In the first Aim, monoclonal antibodies prepared against bone marrow stromal cells, including one against mouse adhesion molecule VCAM1 and new hamster monoclonal antibodies, will be used to localize distribution in intact bone marrow. Employing protocols we have developed to obtain uncultured stromal cells, we will use these antibodies to determine if marrow stromal cells are phenotypically and functionally diverse. Aim 1 will determine if stromal cell-derived cytokines that are important for lymphopoiesis in vitro are indeed produced by stromal cells in vivo. In Aim 2, modifications of the genetic expression and production of stromal cell derived cytokines will be quantitatively assessed during B-lineage regeneration after marrow perturbation by low dose irradiation or treatment with 5-fluorouracil. Expansion of transplanted B-cell precursors into C.B17 scid mice will be used to monitor changes in cytokine production in marrow without ablative regimens. Finally, our novel panel of anti-stromal cell monoclonal antibodies will be used to further discriminate and functionally characterize reticular stromal cells in vivo. The proposed studies are highly related to bone marrow transplantation and leukemogenesis.